Title: Specificity and regulation in a protein kinase cascade affecting the actin cytoskeleton ABSTRACT Eukaryotic cells interpret extracellular and intrinsic cues to effect remodeling of the actin cytoskeleton, a process critical for controlling cell morphology, movement, and invasiveness. Tight control of signaling pathways impinging on the cytoskeleton is therefore essential to normal development and homeostasis. In this proposal we will investigate mechanisms underlying specificity and regulation in protein kinase signaling cascades converging on phosphorylation of the cofilin/ADF (actin-depolymerizing factor) group of proteins, key molecules that mediate remodeling of actin filaments. The RHO family GTPases RHO, RAC and CDC42 each directly activate kinases (ROCK, PAK1 and PAK4, respectively) in a spatially restricted manner that in turn directly phosphorylate and activate LIM kinases. The LIM kinases are exquisitely specific in their ability to phosphorylate cofilin/ADF proteins at Ser3, which inactivates cofilin/ADF by causing their dissociation from actin. This signaling cascade is tightly controlled through multiple mechanisms, including substrate specificity of both the upstream kinases and of LIM kinases themselves, and through autoregulation of the LIM kinases. The major goal of this proposal is to discover the molecular basis for specificity and regulation in this biologically important pathway. In our preliminary studies we have determined the X-ray crystal structure of LIM kinase 1 in complex with its substrate, cofilin. Therefore in Aim 1 we build on this result to test the hypothesis that the exquisite substrate specificity of LIM kinases for cofilin is defined by a novel kinase- substrate interaction by probing biochemical, catalytic and biophysical effects, and functional impact, of disrupting the crystallographically defined LIMK1:cofilin interface. In our preliminary studies we have also begun to map the molecular level details of the autoregulatory head-tail interaction of LIMK1, and so in Aim 2 we will utilize a range of structural, biochemical and biophysical techniques obtain a significantly improved understanding of this head-tail interaction and will then examine the effect of targeted mutations on interdomain interactions, on kinase activity in vitro, and on function in cells. Lastly, in the previous grant period we discovered a novel mechanism for autoinhibiton of type II PAKs through an N-terminal pseudosubstrate sequence. In Aim 3 we will test the hypothesis that type II PAK activation is mediated by direct engagement of this pseudosubstrate sequence by specific SH3 domains, and perform structural, biochemical, and cellular studies of SH3-PAK4 interactions. This aim will thereby provide molecular level details of type II PAK regulation that have remained obscure. In this Multi-PI proposal, the Boggon and Turk laboratories will conduct a highly collaborative structure-directed functional study to provide a significantly improved understanding of the molecular mechanisms that govern LIM kinase-mediated control of the actin cytoskeleton and of the general rules that govern cellular outcomes in protein kinase signaling.